She got a bath
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
White Elephant 01 8.1 burb.
- Thread starter Wes Harden
- Start date
quick update, Rotate tires, flash the pcm, I turned off evap, air inj, and egr ses lamp, in prep to eliminate them.
Update the speedo calibration, read a mile or 2 slow at highway speeds will check against gps on my trip to Pismo next week.
Also did the crank learn procedure, hadn't done it since I changed the cam sensor.
Update the speedo calibration, read a mile or 2 slow at highway speeds will check against gps on my trip to Pismo next week.
Also did the crank learn procedure, hadn't done it since I changed the cam sensor.
truck runs nice and smooth I think the crank relearn made a difference. My speedo adjust however is further off. Before 2.5-3 mph fast now I am 4-5 mph fast.
Goodyear says my tire is 30.7" diameter that's what I put into the VCM tire/gear wizard. A generic 245 75 r16 tire is listed as 30.4 I am going to try that.
Goodyear says my tire is 30.7" diameter that's what I put into the VCM tire/gear wizard. A generic 245 75 r16 tire is listed as 30.4 I am going to try that.
Might need to actually measure from center of hub to ground and find actual diameter. Gonna be shorter under squish
yep, I just looked at my change log, and both diameters are 12 VSS pulses but trans revolutions/mile stat changes. Doesn't help I am just learning this stuff and my retention is crap.
My old school thinking says larger tires read slower then actual, and smaller read faster, so making the tire smaller in the data is making the speedo read faster ? that sound right ?
My old school thinking says larger tires read slower then actual, and smaller read faster, so making the tire smaller in the data is making the speedo read faster ? that sound right ?
with a tape measure and the mark I eyeball with correction, I am getting 14.25, so 28.5. My original tune file shows 29.59, don't think i want to go a whole inch.
Going to try 29.29 and see, not like I can't change it again easy as a flash and test drive.
Going to try 29.29 and see, not like I can't change it again easy as a flash and test drive.
now your gonna make me maths. Yeah I need to do a test drive log file, haven't done that yet.
I thought you were supposed to put the travel distance or circumference not Dia.
Diameter is not accurate because the tire gets a little squish on the pavement so you need so your radius is center to the ground, that number x2 is what the Diameter is
HP Tuners VCM editor gear/tire wizard asks for tire size. The original unedited, at least by my me, log file shows 29.59, I just assumed it wanted diameter, not circumference, since that would 80"+ Now I don't know, more research needed.
The true answer is somewhere in between. Yes, you get squish, but a radial tire still has the same circumference of rubber around it. This means some of the tread has to slip during the rotation. A bias ply tire can actually stretch and grow with speed.
I saw a video before of a guy who tested that. Same tire, went from 50 down to 10 psi. The radius changed by about 2" on only 30" tire, that's a 13% change in radius, but the measured rotations only changed about 1-2%. Now, this is only one test, but it shows the answer is somewhere in between, it's not a s simple as measuring the circumference, or measuring the radius with weight on the tire.
Since we know the gear ratios are fixed, I just change the tire diameter setting, or the PPM, depending on what the ECU uses, until the speedo is correct. The easiest is to do the ratio of error vs the GPS as @Blue85 suggested, you can get spot on pretty quick.
I saw a video before of a guy who tested that. Same tire, went from 50 down to 10 psi. The radius changed by about 2" on only 30" tire, that's a 13% change in radius, but the measured rotations only changed about 1-2%. Now, this is only one test, but it shows the answer is somewhere in between, it's not a s simple as measuring the circumference, or measuring the radius with weight on the tire.
Since we know the gear ratios are fixed, I just change the tire diameter setting, or the PPM, depending on what the ECU uses, until the speedo is correct. The easiest is to do the ratio of error vs the GPS as @Blue85 suggested, you can get spot on pretty quick.
Last edited:
I remember talking with somebody who was limping a vehicle on a mis-sized spare tire. I suppose it must have had a spool. They messed with tire pressure all over the place because the sizes were close, but it made virtually no difference. Think about how a radial tire is made of steel bands within the rubber. No matter what's happening to the side wall, you still have those same # of inches of rubber.
I can imagine, if you can change from 50 down to 10 psi and make almost no difference, within the realm of useable road tire pressure it's pointless.
I thought I would have to correct the speedo a tad on my Regal when I put in the sidewall tire diameter for the speed calculation and not a slightly smaller number for tire squish, but it was spot on to the GPS right out of the gate. They are 28" drag radials. It seems it's closer to that number, than the measured tire radius from the ground.
I thought I would have to correct the speedo a tad on my Regal when I put in the sidewall tire diameter for the speed calculation and not a slightly smaller number for tire squish, but it was spot on to the GPS right out of the gate. They are 28" drag radials. It seems it's closer to that number, than the measured tire radius from the ground.
Last edited:
That is what I did on my big rig when I changed tire size.
I saw the difference in mileage and adjusted the number by that percentage
Attempting to figure the math. Am not sure what data I should be using.
I have a 4mph high reading it appears to constant at all speeds.
This is original data set. With tire diameter 29.59 and 3mph fast on speedo
Also is there away to capture a screen in HP Tunners? Or have two screens of 2 files to compare?
I have a 4mph high reading it appears to constant at all speeds.
This is original data set. With tire diameter 29.59 and 3mph fast on speedo
Also is there away to capture a screen in HP Tunners? Or have two screens of 2 files to compare?
I haven;t used HP tuners, but something is off on your data.
4 MPH error at all speeds would imply an offset in the equation, which is unlikely, because the intercept needs to be zero. It should be a ratio, like 4 MPH at 60 MPH, 2 MPH at 30 MPH, etc. In which case it would be 4/60 = .0667 or 6.67% error
So if the speedo is 4 MPH slow at 60 MPH, you could increase the output by multiplying the current output by 1.0667 (106.7 %)
If the speedo is fast, you could reduce the output by 1 - .0667 = .9333, multiply by .9333.
Make sense?
4 MPH error at all speeds would imply an offset in the equation, which is unlikely, because the intercept needs to be zero. It should be a ratio, like 4 MPH at 60 MPH, 2 MPH at 30 MPH, etc. In which case it would be 4/60 = .0667 or 6.67% error
So if the speedo is 4 MPH slow at 60 MPH, you could increase the output by multiplying the current output by 1.0667 (106.7 %)
If the speedo is fast, you could reduce the output by 1 - .0667 = .9333, multiply by .9333.
Make sense?
Last edited:
You could also just change the tire size accordingly using similar math, if the error gets worse, you simply went the wrong direction.
Last edited:
It is 4 at 60, I triple checked that. I won't bet it's 4 at lower speeds.
I came up with 6.67%, but wasn't sure where to from there.
Wow 30.7 where the tire size was calculated, and 6.67% difference would be 28.65
I will try the 28.65 thanks for your help
I came up with 6.67%, but wasn't sure where to from there.
Wow 30.7 where the tire size was calculated, and 6.67% difference would be 28.65
I will try the 28.65 thanks for your help
I don't remember how my son did the correction in my '01 2500, but we did discover while he was watching HP tuners, that the vehicle speed in the ECM didn't exactly match what I was seeing in the speedometer. I have 265 tires, which is why he made a correction, but I had him leave it showing just shy of 2 mph fast. I would rather have it skewed that way than reading slow as I tend to drive slightly over the speed limit most of the time.
Just a thought.
Just a thought.
